Why do denser things sink? Detailed justification needed I would like a detailed explanation and justification if possible regarding the topic. I would like to know what happens at an atomic level. Most of the answers I have found were very basic and did not explain it thoroughly enough. What I have deduced (which does not feel right) is, a denser object has more particles/atoms, therefore, when the object becomes submerged in water it sinks because it exerts more force per area or pressure (because it has more particles to exert the force), that pressure exerted by the object, is stronger than water's (because water is less dense, therefore less pressure) and it sinks, it will only stop sinking at lower depths when the water's pressure increases. Is this similar with why a bullet is able to pierce (not push) through less dense cotton and not denser concrete.
I am a gr.9 student who is enthusiastic about physics and would like to thank in advance for your responses!
 A: 
What I have deduced (which does not feel right) is, a denser object has more particles/atoms, therefore, when the object becomes submerged in water it sinks because it exerts more force per area or pressure (because it has more particles to exert the force), that pressure exerted by the object, is stronger than water's (because water is less dense, therefore less pressure) and it sinks

This theory is wrong. I will give you a counterexample:
First of all look at the attached picture, 
if $F_1+mg>F_2$ , where $mg$ is the weight of the cylinder, 
then the cylinder sinks. It does not matter what is the cylinder made of. It can be made of a mixture of various atoms of all kind of weights.
In order to disproof your theories we can assume the cylinder (not the jar that contains the liquid) is made of aluminium ($\rho_{Al}<\rho_{Hg}$) and it is empty. You fill it with uranium ($\rho_{U}>\rho_{Hg}$) and you let it sink in mercury ($Hg$). The cylinder will sink despite the fact the mercury is in contact only with the aluminium which is a few times less denser. 

Source
